Technical Field
The present invention belongs to the field of cleaning of solid waste incineration flue gases, and more specifically, to a method for inhibiting the formation of dioxin-like compounds in solid waste incineration flue gases.
Description of Related Art
The incineration technology with the advantages of waste minimization, high efficiency of organic matter decomposition and energy recovery is one of the main means of solid waste treatment. According to the “China Statistical Yearbook 2016”, the quantity of collected municipal solid waste in China was 191 million tons in 2015, with incineration quantity accounting for 32.3%; incineration is also the main way to treat industrial hazardous waste and medical waste, and all provincial-level hazardous waste treatment centers successively set up in the recent decade take incineration as one of the main treatment means. As one of the main byproducts of solid waste incineration, the flue gas contains a variety of gaseous and particulate pollutants. Among them, dioxin-like compounds are persistent organic pollutants, highly toxic and accumulating in the environment. At present, the flue gases are cleaned of dioxin-like compounds mainly by spraying powdered activated carbon before a bag filter which removes dioxin-like compounds by formation and adsorption of a powdered activated carbon film on the surface of the filter bag; however, the removal rate of the method is relatively low, with the maximum not more than 50%, so it is difficult to radically control the dioxin-like compound pollution.
Due to the lack of a high-efficiency cleaning method at the end, it is necessary to explore a technology which can control dioxin-like compounds in flue gases at the source. According to the research in recent years (Advancement in Study on Formation Mechanism of Dioxin in MSW Incinerators, Thermal Power Generation, 34 (2005), 15-20, by Cao Yuchun, Yan Jianhua, Li Xiaodong, Chen Tong and Cen Kefa), dioxin-like compounds may be formed in four ways during the incineration of solid waste: 1) The release of dioxins that originally exist in the solid waste in the incinerator; 2) the gas-phase synthesis of dioxins at a high temperature in the incinerator; 3) the formation of dioxins after the incinerator through de novo synthesis when the flue gases cool down to a specific temperature range; and 4) the formation of dioxins after the incinerator through synthesis of precursors when the flue gases cool down to a specific temperature range. In the ways mentioned above, dioxins are not industrial products and solid waste originally contain an extremely low amount of dioxins, so the contribution of the first way to the formation of dioxins in incineration is almost negligible; both the second and the fourth ways take chlorophenol, polychlorinated biphenyl and other organic chlorinated compounds as raw materials for dioxin synthesis, and these organic chlorinated compounds have an extremely high conversion rate in modern solid waste incinerators with strict temperature, material mixture and gas retention time control measures and the content of the dioxins in the flue gases is extremely low, so the two ways are unlikely to become the main contributor of dioxin formation in incineration due to the restriction of the amount of raw materials; while the third way has currently been identified as the main way of dioxins formation in incineration flue gases and should be the key link for controlling flue gas dioxins at the source for there is no restriction by raw materials of dioxin synthesis, as it takes simple materials (CO and HCl in the flue gases, and carbon residues in particulates, etc.) as raw reactants. Therefore, according to China's “Technical Specifications for Centralized Incineration Facility Construction on Hazardous Waste (HJ/T176-2005)”, the high-temperature flue gases produced by waste incineration shall be cooled down to below 200° C. within 1.0 s through rapid cooling treatment so as to shorten the retention time of the flue gases in a temperature range of 200˜500° C.; this temperature range is an appropriate temperature interval for the de novo synthesis of dioxin-like compounds. But the cooling method by direct heat exchange of water and high-temperature flue gases that is adopted in the rapid cooling treatment is restricted by the concurrent transfer rate of heat and mass, so it is actually difficult to achieve the required cooling rate; at the same time, the rapid cooling treatment makes it impossible to recover the residual heat of the flue gases (power generation), so the method cannot be applied in the solid waste incineration relying on power generation to compensate costs.
Chinese Patent No. CN 101766951A discloses a system for inhibiting the formation of dioxins in waste incineration, comprising an incinerator, a cooling unit and a first separating unit which are sequentially connected, wherein the cooling unit is used for recovering some heat energy and cooling the flue gases containing fly ash generated in the incinerator to a temperature range of 400˜800° C., and the first separating unit is used for the removal of fly ash containing chlorides, organic chlorides and heavy metal particles from the flue gases after the cooling treatment in the cooling unit, as well as the removal of transition metal cations which acts as catalysts in dioxins formation. This invention inhibits or reduces the formation of dioxins by a physical ash-removing method only, and has a poor inhibition effect.
Chinese Patent No. CN 204478079U discloses a rapid cooling unit for inhibiting the re-synthesis of dioxins in flue gases in a low-temperature pyrolysis incinerator, wherein the flue gases enter a flue gas inlet end from the bottom of the rapid cooling unit, evenly flows upward through a grating and over heat exchange carriers, and finally goes out from a flue gas outlet end; a cooling water circulation system comprises a cooling water machine, a water pump, a cooling water spray device and a backwater tank, wherein the flue gas flow lines are upward and parallel to each other, the cooling water is sprayed on the heat exchange carriers to cool the flue gases, and the flue gases and the cooling water flow in a opposite direction, to rapidly cool the upward flue gases from 700° C. to below 200° C. within 2 s, but the cooling method by direct heat exchange of water and high-temperature flue gases that is adopted in the rapid cooling treatment is restricted by the concurrent transfer rate of heat and mass, so it is actually difficult to achieve the required cooling rate; at the same time, the rapid cooling treatment makes it impossible to recover the residual heat of the flue gases (power generation) to be implemented, so that the method cannot be applied in the solid waste incineration relying on power generation to compensate costs.
The de novo synthesis of dioxin-like compounds is a catalytic process, and a large number of studies have revealed that the key catalyst in this process is copper chloride (CuCl2) which is more than ten times other known catalytically active substances in the capability of catalyzing the formation of dioxin-like compounds (Influencing Factors and Control Means of De novo Synthesis of PCDD/F, Chemical Industry and Engineering Progress, 25(2006), 557-562, by Ma Hongting and Zhang Yufeng), wherein the main role of CuCl2 in catalyzing the formation of dioxin-like compounds is direct decomposition to provide chlorine atoms for carbon atoms to form C—Cl bonds; form chlorinated aromatic compounds via C—C bond cleavage and then form tricyclic chlorides (dioxin-like compounds) via condensation and other reactions. Obviously, converting CuCl2 contained in the incineration flue gas into a catalytically inert substances before reaching the appropriate catalytic reaction temperature range (200˜450° C.) is an effective method to inhibit the formation of the dioxin-like compounds in solid waste incineration flue gases.